An IP multimedia subsystem (IMS) has attracted attentions as a core communication system for providing a new service in an IP (Internet Protocol) based nextgeneration communication environment. The IMS provides a variety of multimedia services in an environment of mobile communication networks and selves to unify IP networks and mobile communication environments as a set of nodes for performing a call control. Accordingly, the IMS makes it possible to provide an IP-based multimedia service using user equipments by facilitating interoperations between the user equipments and the IP networks. The IMS includes three layers of a connectivity layer for connection to the IMS, a control layer for managing a call and session routing, a charging process, and a file protocol, and an application layer for storing and managing data and generating a service for a subscriber.
The IMS can support a variety of contents types such as voices, multimedia data, and texts in interoperation with the PSTN as the existing telephone network and the Internet. Since the IMS employs a session initiation protocol (SIP) as a signaling protocol usable for the IP-based multimedia service such as an Internet telephony service, a remote video conference service, a voice mail service, and a location service, the IMS can support the existing VoIP and a real-time streaming service. The SIP is a call control protocol of the application layer for retrieving a location of an opposite user equipment and generating, correcting, and ending an IP-based multimedia service session between user equipments or between a user equipment and an entity having an IP address.
On the other hand, the mobile communication systems has a relevant function unit for calculating a location of a terminal and provides a location service of calculating location information of a target terminal and forwarding location information including the calculated location information to an entity in the network. Examples of the method of calculating the current location of the target terminal can include a cell-ID method using an ID of a cell to which the terminal belongs, a method of measuring a period of time when a radio wave travels to a base station from the terminal and then calculating a location of the terminal using a trigonometrical survey, and a method using a global positioning system (GPS).
The location service using the mobile communication system serves as a basis for providing users with an emergency location service as well as a variety of accessory services such as a traffic information service, a location information service, a weather information service, and a car navigation service. Particularly, since the emergency location service (for example, a service for emergently establishing an emergency call between a user equipment and an emergency center when a user is subjected to a disaster or a traffic accident) require rapid check of a location of the user to establish an emergency call with an accurate emergency center, the emergency location service needs to provide current location information or updated location information of a user equipment to provide an emergent rescue to the user, as well as to establish an emergency call between the user equipment and an emergency center most suitable for the current location of the user equipment by the use of a system including sets of mobile communication networks or control nodes.
However, the structure of a network involved in the emergency location service varies depending on network structures to be supported and call processing procedures and transmission parameters between entities in the network for providing the emergency location service are different from each other. Therefore, a method for providing an emergency location service using interoperability between them is required.